Quality parameters of photomask-forming silica glass substrates include the size of defects, the density of defects, flatness, surface roughness, the opto-chemical stability of material, and the chemical stability of surface. Of these parameters, the quality relating to the flatness on substrates is more strictly required to comply with the trend of ICs toward finer-circuitry. It is highly probable that the design rule of interconnection to be transferred to silicon wafers will fall below 100 nm. Then, silica glass substrates for photomasks must have a flatness below 0.3 μm for the 6025 substrate (152 mm by 152 mm by 6.35 mm). Specifically, when a wiring pattern on a photomask is optically transferred to a silicon wafer, the exposure surface is desired to be as flat as possible. The flatness of the exposure surface is correlated to many factors including the material and thickness of a light-shielding film, the type of wiring pattern and the location within the exposure system although the flatness of glass substrate is one of predominant factors. If the design rule of interconnection is reduced to below 100 nm in the future and the 6025 substrate yet has a flatness in excess of 0.3 μm, then the critical dimension (CD) accuracy of the wiring pattern transferred to the silicon wafer exceeds the permissible range. This leads to a failure to establish fine-circuitry devices.
The current technology of leveling or flattening silica glass substrates for photomasks is an extension of the traditional polishing technology. The surface flatness achievable with the current technology is on the average about 0.5 μm at best for the 6025 substrate. Substrates having a flatness of less than 0.5 μm can be obtained, but in very low yields. This is because in the traditional polishing technology, it is possible to roughly control the polishing rate over the entire surface of a substrate, but it is impossible in a practical sense to specify a leveling recipe for each of starting substrates in accordance with their shape and individually polish the substrates for evening out irregularities. Also, when double-sided lapping on a batchwise basis is used, for example, it is very difficult to control variations within each batch and between batches. When single-side lapping on a single-wafer basis is used, a difficulty arises in that variations occur in conformity to the shape of starting substrates. In either case, it is difficult to consistently manufacture substrates with improved surface flatness.